Overcooling in the upper thermosphere during the recovery phase of the 2003 October storms

Infrared radiative emissions by carbon dioxide (CO₂) and nitric oxide (NO) are the major cooling mechanisms of the lower thermosphere. During geomagnetically active periods, the NO density and cooling rate in the auroral regions increase significantly as a result of particle precipitation and Joule heating. Previous studies have shown that the time for NO density to recover to quiet time levels is longer than that of the thermosphere temperature or density recovery. This study explores the implications of these different recovery rates for the post-storm thermosphere. Thermosphere densities retrieved from the CHAMP and GRACE accelerometer measurements and NO cooling rates measured by TIMED/SABER are used to examine their variations during the post-storm period of the October 2003 geomagnetic storms. It was found that thermosphere densities at both CHAMP and GRACE altitudes recovered rapidly and continuously decreased below the quiet time densities during the post-storm period, especially at middle latitudes. Compared with the quiet time values, the maximum depletion in the CHAMP and GRACE densities after the storm is about 23-36%, and the estimated decrease of thermospheric temperature is as large as 70-110 K. Our analysis suggests that the elevated NO cooling rate, resulting from the slower recovery of NO densities in the post-storm period, is a plausible cause for this apparent post-storm overcooling of the thermosphere.